Some here on BYC have chickens with only eye Mareks who seem to do okay. Generally up to 100% of chickens can die of Mareks within 10 weeks. There are a couple of ongoing threads about Mareks in which members discuss what they are going through. It is a good way to get tips on what to expect and other information about testing. Here are a few of those, along with some veterinary school or professional articles:
https://www.backyardchickens.com/t/741957/not-an-emergency-mareks-in-the-flock
https://www.backyardchickens.com/t/894589/moving-forward-breeding-for-resistance-to-mareks-disease
https://www.backyardchickens.com/t/752961/help-mareks-disease
https://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://www.merckvetmanual.com/mvm/poultry/neoplasms/mareks_disease_in_poultry.html
https://www.backyardchickens.com/t/741957/not-an-emergency-mareks-in-the-flock
https://www.backyardchickens.com/t/894589/moving-forward-breeding-for-resistance-to-mareks-disease
https://www.backyardchickens.com/t/752961/help-mareks-disease
https://extension.unh.edu/resources/files/Resource000791_Rep813.pdf
http://www.merckvetmanual.com/mvm/poultry/neoplasms/mareks_disease_in_poultry.html